This invention relates to a method and apparatus for measuring electrical characteristics of an electrical system and, in particular, the characteristics of an electrical energy delivery system. The invention is particularly adapted for evaluating characteristics of a vehicle electrical system, but has applications in many different types of electrical systems.
In many test settings, it is useful to be able to determine circuit parameters, such as resistance, total energy source and cable performance, and current flowing in a circuit. This is especially useful for troubleshooting electrical problems in vehicles, such as automobiles, trucks, boats and the like. Very often components, such as batteries, starter motors, alternators, and the like, are returned under warranty to the manufacturer, only to find that they are not defective.
Presently, the common method of assessing battery condition is to connect a voltmeter to the terminals of the battery, to apply a small value resistor capable of dissipating a large amount of heat to the battery terminals, and to observe terminal voltage as a function of time while the battery discharges. This test permanently changes the battery, the test cannot effectively be performed unless the battery is fully charged, and the results must be compensated for battery temperature in order to be accurate.
Another alternative for testing a battery is to apply a small AC signal to the terminals of the battery, and use a Wheatstone (or one of its derivatives) bridge to measure the internal resistance of the battery. Battery internal resistance is related to battery condition, and this test has the advantages of allowing a relatively small test apparatus that does not materially affect the condition of the battery, and which can be applied to batteries that are less than fully charged.
In many cases, a conventional ohmmeter is useless in determining wire and connection resistance because the resistance is too small. Choosing the example of an automobile electrical system, approximately 200 amperes are required to crank the engine. In this case, resistances as small as 10 milliohms are a significant problem, but well below the range of a conventional ohmmeter. In this case, the usual recommended procedure is, again, to attach a low value resistance to the system, drawing a large current through the wire(s) and/or connection(s) to be tested, and quickly measuring the voltage drop(s) to infer system resistance.
The measurement of current is usually accomplished in one of two ways: an in-circuit ammeter, or a clamp-on device that infers current from the surrounding magnetic field.
Insertion of an in-circuit ammeter requires that the circuit be broken, and the meter inserted, so that the current to be measured flows through the meter. This has the disadvantage of being awkward to do. It is also usually limited to currents below 50 amperes, by the practical constraints of meter and connector resistance.
Clamp-on current probes that infer current from the magnetic field surrounding a current are more convenient. However, inexpensive models offer only limited accuracy and are unable to measure currents below roughly 10 amperes with acceptable accuracy. Accurate models tend to be quite expensive.
There is no commonly available means of making a one-step assessment of a system consisting of an electrical energy source and conductors used to deliver the electrical energy to a destination.
The apparatus described here has many applications, including tests of vehicle and stationary batteries, measurement of resistances as small as a fraction of a milliohm, one-step appraisal of a system consisting of a source of electrical energy and connecting cables, and measurement of current flowing in a circuit, all using the same basic circuit.